Introduction to CPU

1. Introduction of CPU
• The part of the computer that performs the
bulk of data-processing operations is called
the central processing unit.
• The CPU is made up of three major parts:
The register set
Control Unit
ALU

2. • The register set: It stores intermediate data used
during the execution of the instructions.
• The arithmetic logic unit (ALU): It performs the
required microoperations for executing the
instructions.
• The control unit: It supervises the transfer of
information among the registers and instructs the
ALU as to which operation to perform.

3. Major components of CPU

4. Computer architecture
• Computer architecture is defined as the
computer structure and behavior as seen by
the programmer that uses machine language
instructions. This includes the instruction
formats, addressing modes, the instruction
set, and the general organization of the CPU
registers.

5. Control Memory
• The function of the control unit in a digital
computer is to initiate sequences of
microoperations.
• When the control signals are generated by
hardware using conventional logic design
techniques, the control unit is said to be
hardwired.

6. • A control unit whose binary control variables
are stored in memory is called a
microprogrammed control unit .
• The microinstruction specifies one or more
microoperations for the system.
• A sequence of microinstructions constitutes a
microprogram.

7. • A computer that employs a microprogrammed
control unit will have two separate memories:
a main memory and a control memory.
• The main memory is available to the user for
storing the programs.
• The contents of main memory may alter.

8. • In contrast, the control memory holds a fixed
microprogram that cannot be altered by the
occasional user.

9. • The user who programs the computer in
machine or assembly language must be aware
of the register set, the memory structure, the
type of data supported by the instructions,
and the function that each instruction
performs.

10. General Register Organization
• We know that memory locations are needed
for storing pointers, counters, return
addresses, temporary results, and partial
products during multiplication.
• It is more convenient and more efficient to
store these intermediate values in processor
registers.

11. • Hence it is necessary to provide a common
unit that can perform all the arithmetic, logic,
and shift micro operations in the processor.

12. Microprogrammed control
organization.

13. A bus organization for seven CPU registers is shown in Figure

14. For example,
• To perform the operation R 1 <-- R2 + R3 the
control must provide binary selection variables to
the following selector inputs:
1. MUX A selector (SELA): to place the content of
R2 into bus A .
2 . MUX B selector (SELB): to place the content o f R
3 into bus B .
3 . ALU operation selector (OPR): to provide the
arithmetic addition A + B .
4. Decoder destination selector (SELD): to transfer
the content of the output bus into R1 .

15. Control Word
• There are 14 binary selection inputs in the
unit, and their combined value specifies a
control word. The 14-bit control word is
defined as:

16. • It consists of four fields.
• The three bits of SELA select a source register for
the A input of the ALU.
• The three bits of SELB select a register for the B
input of the ALU.
• The three bits of SELD select a destination
register using the decoder and its seven load
outputs.
• The five bits of OPR select one of the operations
in the ALU.

17. The encoding of the register selections is specified in
given Table

18. In addition, the CPU must provide shift operations. The encoding
of the ALU operations for the CPU is specified in given Table:

19. Examples of Micro operations
• For example, the subtract micro operation
given by the statement
R 1 <- R 2 - R3
• A control word of 14 bits is needed to specify
a micro operation in the CPU.

20. The corresponding binary value for each
field(subtract micro operation) is as follows:

22. • The direct transfer from input to output is
accomplished with a control word of all 0' s
(making the B field 000).
• A register can be cleared to 0 with an
exclusive-OR operation. This is because x EXOR
x = 0.

23. • A memory unit that stores control words is
referred to as a control memory.
• By reading consecutive control words from
memory, it is possible to initiate the desired
sequence of Micro operations for the CPU.
This type of control is referred to as micro
programmed Control.